Giles et al,
I don't think you will ever get your preferred phraseology adopted at Heathrow because of the interaction between such things as Helicopter Routes, London City, White Waltham etc. and the approach. It can be a pain I admit, but I think that the "cleared ILS" phraseology would end up generating more paperwork in the long run, especially as speed control is vital and some pilots may assume that the clearance removes the ATC imposed speed.
This particular item has been covered here before.
There is r/t phraseology available at Heathrow, and all other UK airports, that allows what is being requested. Just a shame more don't use it.
We are allowed to say "when localiser established descend on the ILS".
At Heathrow it would be along the lines of "turn left heading 300 degrees report localiser established 27L" as normal then when appropriate
and before the arrival has reported loc established "descend to altitude 3,000ft, when localiser established descend with the ILS". It's also recommended these days, following instances of a/c slowing unexpectedly, to say "maintain speed xxx knots" when giving descent on the ILS.
It's a bit more cumbersome than just saying "cleared ILS" but it works and also protects the numerous a/c and helicopters that operate under the approaches.
WF.